drop min certbot coverage (#7972)

`tox -e cover` fails for me on macOS. This is due to the differences in the code that is run when on Linux vs. other platforms in `certbot.util` and its tests. Diffing my local coverage with Travis, the only difference in lines missing test coverage is:
```
$ diff travis.txt local.txt
< certbot/certbot/util.py                                        238     24    90%   132-134, 210, 275-281, 318, 330-340, 347, 381-382
> certbot/certbot/util.py                                        239     34    86%   30, 132-134, 210, 275-281, 301, 305, 316-318, 330-340, 347, 367-373, 381-382
< certbot/tests/util_test.py                                     392      0   100%
> certbot/tests/util_test.py                                     375     26    93%   483-487, 492-502, 507-514, 548-550, 555-557
```
I think tests on `master` should not be failing locally for people.

While there would be other ways to fix this by adding `# pragma: no cover` lines or writing mocked out tests for other platforms, I personally just think dropping the minimum coverage one percentage point is fine at least for now.

tox.cover.py

@@ -14,7 +14,7 @@ DEFAULT_PACKAGES = [
     'certbot_dns_sakuracloud', 'certbot_nginx']
 
 COVER_THRESHOLDS = {
-    'certbot': {'linux': 96, 'windows': 96},
+    'certbot': {'linux': 95, 'windows': 96},
     'acme': {'linux': 100, 'windows': 99},
     'certbot_apache': {'linux': 100, 'windows': 100},
     'certbot_dns_cloudflare': {'linux': 98, 'windows': 98},